feat: Add JSON import / export of source and translation text

[spaghetti-stack]

Changes

• Allows exporting source and translation text of a whole project to JSON format.
• Allows importing translation text for a whole project in JSON format.

This way it is now possible to use an external translation provider to, for example:

• Perform translations using an LLM that make use of the full context of all pages.
• Use external translators not currently supported by Koharu.
• Compare and switch between multiple translations without having to rerun the translation pipeline.

This feature is similar to #614 and #555 (but without speaker ID).

I chose JSON as format because it is easy to parse, and the format is (slightly) more compact than XML so using an LLM for whole context translation would use less tokens. XML or Markdown could also be added in the future.

User facing changes:

Added the "Export source texts", "Export translations" and "Import translations" buttons to the UI which are used to interact with this new feature.

Testing

I have manually tested the import / export functionality. There are also front end and back end unit tests.

[github-actions]

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